Paul told the Galatians that they had been running a good race and making great time. But something happened to them so in Galatians 5:7 he asks, “Who tripped you up?” Why did they let themselves get hoodwinked away 
from Christ and back to the law? It seems that there aren’t a lot of Judaizers, just a few very persuasive ones. Verses 8-9 say, “This persuasion is not from Him who calls you. A little leaven leavens the whole lump.” I’m confident that these legalists came in the guise of friendship and wooed the Galatians by expressing their concern for their welfare. “We only have your best interest in mind,” they might say as they turn young Christians away from grace and freedom in Christ toward the binding chains of legalism. You’ve heard the saying, “Lord, deliver me from my friends; I can take care of my enemies myself.”
We know where that kind of persuasion comes from. Or better, we know where it doesn’t come from! It certainly isn’t from God. It’s just a little bit of “yeast” in the lump that is causing all the problems. Where does this yeast come from? In 1 Corinthians 5:6, Paul refers to yeast also. He speaks of its penetrating power. Jesus also warned His disciples to watch out for the “yeast” of the Pharisees (Matthew 13:33). Boice says, “The point is simply that the doctrine of salvation by works is not of God but rather proceeds from that which is hostile to God’s grace.” The UBS handbook for translators says, “The meaning of the proverb is fairly obvious: evil, no matter how small it seems, will always in the end result in great harm. Paul may be applying the proverb either to the teachers, who obviously were only a handful, or to their teaching, especially to their possible insistence on circumcision as only a small thing.”
I like the way Richison applies this passage. He writes, “It only takes a little false doctrine to ruin a local church. A speck on the telescope will distort the heavens. Benjamin Franklin said, ‘For want of a nail the shoe was lost; for want of a shoe the horse was lost; for want of a horse the rider was lost; and for want of a rider the battle was lost.’ Any teaching that adds to Christ’s work on the cross, no matter how small, does damage to His work of grace. Grace plus any work, no matter how small, is evil leaven.”